2 Answers
2

To make a new partition, first shrink an existing partition (if you don't have enough unallocated space). The remaining space would be turned into unallocated space. Then you can make a new partition in that space.

After that, you can normally install your Windows. But you won't be able to boot back into Ubuntu as the GRUB, needed to boot Ubuntu, would be replaced by the boot loader of Windows.

You would need a Live USB/CD/DVD of Ubuntu. After installing Windows, boot with any Live media. Open up a terminal and run the following commands:

sudo grub-install /dev/sda
sudo update-grub

Note that here /dev/sda is the name of the harddisk. To check the name of your harddisk run sudo fdisk -l. You'll see several names like /dev/sda1, /dev/sda2, etc. These are the names of the partitions, but /dev/sda is the name of the disk.